Explosive devices

ABSTRACT

An explosive device is provided, containing an explosive formulation or explosive ink, which is capable of being disposed of onto a substrate for the device by well known printing and depositing techniques, such as screen printing, ink jet printing or gravure methods. The formulation contains an ink resin binder, a metal and a non-metal in particulate form where the diameter of the particles is less than 10 μm, such that when the formulation is heated, a reactive output results. The substrate can be chosen from any inert material or alternatively an energetic material for which the formulation provides a means of initiation. Preferred metals are aluminum iron or titanium and non-metals are carbon, silicon, boron or metal oxides such as copper oxide, nickel oxide or molybedenum oxide. Devices according to the invention may take the form of a; pyrotechnic train, initiators, fuseheads, initiators for gas generators, microthrusters, and may form part of a larger system containing energetic materials.

This invention relates to explosive devices and to methods of producing such devices particularly those containing an insensitive explosive formulation which has been printed onto a substrate.

Energetic munitions, such as flares, missiles etc require three distinct components; an initiator, an explosive train to amplify the output from the initiator and a main output charge, which can be a high explosive, pyrotechnic or propellant. The method of processing, for example a pyrotechnic, usually requires that the main output charge be consolidated into a pellet to which is applied a priming explosive which is either painted on as a thick slurry or alternatively is consolidated onto the pellet in the form of a final increment. Finally an initiator is attached to the priming explosive. Usually the initiator is itself enclosed in a sheath and is mechanically attached to the primed end of the pellet or pellet housing. Further it is desirable if there is intimate contact between the initiator and the priming explosive and/or main charge otherwise the device may not function. It would therefore be useful tQ be able to deposit or print the initiator or indeed the explosive train directly onto the consolidated explosive to ensure good contact.

The term explosive as used in this specification and claims includes all types of pyrotechnic and gas-generating systems and the terms explosive device and explosive composition should similarly be construed to include pyrotechnics, gas generators, delay lines and similar energetic systems as well as high explosives. It will be well understood that the types of energetic outputs from the explosive devices may include burning, deflagration or detonative events.

Energetic formulations are typically made from at least 95-99% of an energetic material, with the remaining portion being made up of an inert binder. The binder is used to help with the consolidation of the formulation into a pellet. If an excess of binder, is used the formulation will reach a level where there is too little output or more likely the formulation will fail to sustain an exothermic reaction. Consequently conventional depositing or printing techniques rely on the formulation being applied as a thick slurry or paste.

GB Patent No. 2049651 in the name of Brock's Fireworks Limited describes a process for applying a pyrotechnic or explosive composition to a surface. The process comprises screen printing the composition in the form of a liquid, slurry or paste, onto a surface, and allowing the composition to dry or harden.

Although conventional screen printing techniques are used, it is preferred to use a course mesh with the printing screen since this not only enables a thick layer of composition to be printed, but also allows relatively large solid particles to pass through without becoming clogged.

The Brocks patent does not provide any guidance on particle sizes to use; however the energetic materials used there in are typically highly sensitive materials such as silver fulminate, or nitrate/chlorate based mixtures such as SR252. Military compositions such as SR 252 disclosed therein conventionally use particle sizes approximately 20-50 μm in diameter. Although finer powders may be used in conventional devices there is a general reluctance to produce an explosive formulation with smaller diameter particle sizes, as the reactivity of the mixture will be thereby increased, due to the large surface area. Consequently a balance between reactivity and safety is generally sought, using particle sizes in the range mentioned.

In order to print or deposit an explosive formulation the formulation must be; free flowing, devoid of large particulates to avoid clogging, reproducible, and should be relatively safe, both for handling and when deposited. The relationship between safety and small particle size is the main obstacle to overcome. As mentioned above, fine powders can be extremely reactive and so are generally avoided. However, to achieve a formulation which can be used for printing through a fine mesh or nozzle, small diameter particles are required.

A further obstacle to be overcome is ensuring that the final energetic device meets insensitive munitions requirements. Typically the initiator part of the device is considered to be the most sensitive part and therefore it would be desirable that the formulation once deposited should not provide such a vulnerable ‘weak link’ in the final device.

Consequently there is a requirement for explosive formulations which contain small diameter particles, are sufficiently free flowing to allow a finer mesh to be used in a method of depositing the formulation by printing, without causing any blockage of the printing device or to allow alternative deposition methods to be used successfully and which yet demonstrate a reasonably low level of hazard.

It is an object of the present invention to provide such formulations and moreover to provide a more adaptable and efficient process for depositing the formulations in order to produce explosive devices

According to the present invention, there is provided an explosive device comprising a substrate on which one or more explosive formulations is deposited, wherein the one or more formulations comprises a binder, at least one metal and at least one non-metal, wherein the non-metal is selected from a metal oxide or any non-metal from Group III or Group IV, characterised in that the metal and non-metal particles are 10 μm or less in diameter.

Further, it may be desired to increase the flow of the formulation, which can be achieved by using finer particle sizes, and therefore the metal and/or non-metal particles may be 1 μm or less in diameter or even 0.1 μm or less in diameter. The typical upper limit for the particle size for the metals is of the order of one to two microns, while for the non-metals it is of the order of 5 microns.

The metal may be selected from any metal commonly used in pyrotechnic compositions, preferably the metal may be selected from aluminium, titanium or iron.

The non-metal may be selected from any Group III or Group IV non-metal, and is preferably silicon, carbon or boron. The metal oxide may be selected from any metal oxide commonly used in pyrotechnic compositions, being preferably copper oxide, molybdenum oxide or nickel oxide.

The metal and metal oxide will be chosen such that, when initiated, the formulation will give a favourable exothermic reaction. It will be clear to the skilled man as to the required ratios of the at least one metal and non-metal in order to achieve a sustainable exothermic reaction. One convenient selection of the ratio of metal to non-metal may be a stoichiometric ratio. However, fuel-lean and fuel-rich ratios may also be selected. Further it is clear that the metallic element from the metal and metal oxide should not be the same, as there would in that case be no exothermic reaction.

The hazard response from an explosive composition can be characterised in many different ways, such as by subjecting it to a dropped mass, to give a figure of insensitiveness (F of I). In another test the composition can be rubbed between two surfaces giving a figure of friction (F of F), and additionally it can be tested by subjecting the composition to an electric spark to find the level of energy at which the composition ignites. Finally the composition can be heated to ignition to determine its thermal stability.

Typically a very reactive pyrotechnic will have a low figure of insensitiveness in the range of from 20 to 30. A low F of I number indicates a hazardous material and therefore typically some pyrotechnics are classed as very sensitive. By comparison the majority of high explosives have an F of I above 80 and these would be classed as relatively safe. Similarly for the figure of friction a low number indicates a sensitive material. It would be expected that a reactive pyrotechnic formulated from particles having a diameter of a few microns, such as in the present invention would have an F of F of less than 3, whereas an insensitive composition would have a number greater than 6.

The deposited formulation in the device of the present invention may comprise an insensitive explosive composition having a figure of insensitiveness greater than 60; preferably the figure of insensitiveness is greater than 150. The deposited formulation preferably has a figure of friction greater than 4, most preferably greater than 6.

The deposited explosive formulations in the device of the invention comprise a mixture of metal powder and non-metal powder in a binder, which acts as both a binder and as a carrier for the formulation aiding its deposition on the substrate. The binder, will be present in the range of from 10% to 50% by volume of the formulation, preferably in the range of from 30% to 40% by volume, the actual amount being chosen by the application for which the finished device is intended. The selection of binder may also depend on the substrate to be coated.

The large percentage volume of binder ensures that the high surface area metal and non-metal particles are sufficiently dispersed such as to cause the formulation to be relatively insensitive to external hazard stimuli. However, due to the very much increased reactivity of the small diameter, large surface area, metals and non-metals the applicant has advantageously found that the formulation, when initiated is capable of sustaining an exothermic reaction. The small particle size allows for an increased level of homogeneity of the components.

Although relatively insensitive, the mixtures used in devices according to the invention are nevertheless capable of sustaining an exothermic reaction when suitably initiated, a convenient method of ignition may be an explosive primer or electric ignition source.

Binders used to prepare the formulation may be any commercially available ink resins, such as a two part polyurethane such as Nylobag® or a two part epoxy resin such as Polyscreen®.

In particular applications, where a greater output of energy is required from the explosive formulation, energetic binders may be used to provide part of the binder component. Such binders include Polyglyn (Glycidyl nitrate polymer), GAP (Glycidyl azide polymer), or Polynimmo (3-nitratomethyl-3-methyloxetane polymer).

Preferred combinations of the explosive formulation include titanium and boron, and titanium and carbon; Further aluminium with molybdenum oxide, nickel oxide or copper oxide can be used, or iron can be substituted instead of aluminium. These metal/non metal combinations offer increased energy output when the explosive formulation is ignited. Additionally silicon may be used as a “metal” although in such cases the “non-metal” may not be silicon, but may be, for example, carbon or boron.

Pyrotechnic formulations when ignited produce light, heat, sound, smoke and gas, the relative amounts of each output depend upon the specific properties of the formulation. Consequently the formulation may be selected-depending on the application required. Examples of such applications may be, initiators, ignitors or detonators, explosive delay timers, explosive trains, thrusters, sound, smoke or light producers, actuators or heaters.

The substrate onto which the explosive-formulation is deposited can be either inert wherein the substrate is for example a polyester, polyimide, paper, Nomex paper, PET, polystyrene or ceramic. Preferably polyester or polyimide is chosen as they offer good compatibility with the binder and its drying or curing requirements. Alternatively the substrate can be a surface of a consolidated explosive, such as a pressed pellet of explosive material or melt cast explosive, such that an explosive track is deposited directly onto the consolidated material. It would be clear to the skilled man that the consolidated material may be a pyrotechnic, propellant or high explosive.

It may further be desirable for the substrate to possess one or more voids, or elongated voids such as channels or grooves, where the explosive formulation fills the void or elongated void.

It may be desirable to cover the deposited formulation with a protective layer or indeed another substrate to build up a plurality of devices.

Initiators are commonly activated by passing sufficient current through an ohmic resistor, such as a thin metal bridgewire, to cause a localised heating effect of the ohmic resistor, to cause the initiation of the energetic material deposited thereon. It may be desirable to pre-form such a bridgewire or ohmic resistor directly onto the substrate, either by lithographic etching or by a metal printing method as set out in the published applications GB0113408 and GB0128571, such that when the explosive ink is printed, an amount of said energetic formulation lies over part of the ohmic resistor. It will be clear that substantially complete electrical circuits comprising a plurality of ohmic resistors may be deposited onto a substrate, prior to the deposition of the energetic formulation on said ohmic heaters.

It will be clear that one or more surfaces of the substrate as herein before defined may have plurality of energetic formulations deposited thereon and optionally a plurality of said electrical circuits to initiate said deposited energetic material.

Energetic ingredients may be added to the explosive formulation mixture to improve the energy output. Such ingredients are potassium dinitrobenzofuroxan (KDNBF), barium styphnate, and cyclo-1,3,5-trimethylene-2,4,6 trinitamine (RDX) for example. However, it should be noted that these ingredients may cause the deposited formulation to become more sensitive.

Additionally Group I or Group II metal salts, where the counter ion may be selected from any inorganic or organic counter ion, may be added to the deposited explosive formulation to produce in use, a coloured light or sound emission, which may be desirable for the pyrotechnic effects industry.

According to a further aspect of the present invention there is described a method of producing an explosive device comprising the steps of:

-   a) mixing a portion of a binder with at least one metal in the form     of particles having a diameter of less than 10 μm; -   b) mixing a further part of the binder with at least one non-metal,     wherein the non-metal is selected from a metal oxide, or any     non-metal from Group III or Group IV in the form of particles having     a diameter of less than 10 μm; -   c) mixing together the products of a) and b) to provide an explosive     formulation; -   d) depositing the formulation so produced onto a substrate; and -   e) allowing the formulation to dry on said substrate.

It will be clear to a person skilled in the art of pyrotechnic formulations that the above components of the composition could be combined in one step. However it is very desirable to leave the combining of the metal and non metal to the last possible moment to reduce the risk of accidental initiation. Further the part-solutions may be safely stored long term without extensive explosive precautions.

Using an explosive ink or formulation with such a small active particle size means that the ink has extremely good flow properties, and can be printed with screen printing or nozzle-jet techniques with minimum clogging or waste of material. Dot-matrix printing, rotary gravure printing techniques, brushing, dipping or spraying can also be used. Furthermore, the ink or formulation may be substantially free of lead, which means that it may be safer to manufacture and use.

Preferably, a nozzle-jet printing apparatus is used, as this allows any pattern of ink to be printed directly onto a substrate by movement of the printing head. No templates are required.

However in certain instances it may be desirable to have an explosive formulation which is considerably less viscous than the formulation containing only a binder/resin, such as when it is desired to apply the formulation by a wet printing method. In such cases the formulation is required to be in the form of an ink and this can be produced by mixing together the formulation as previously discussed with any commonly used printing solvent, such as a volatile organic solvent. The volatile organic solvent may be selected from a lower alkyl alcohol, ketone or ether or from petroleum ethers ranging from C5 to C10.

As to the amount of solvent required it will be clear to a person skilled in the art of printing techniques as to the desirable viscosity range for any given printing method.

Printing such an explosive ink onto a substrate by means of ink jet, or dot matrix printers has certain advantages due to the ease by which the printing pattern can be changed. Unlike screen printing, a single ink jet or dot matrix printer, controlled by a computer, can produce different printing patterns of explosive ink on the substrate in a single production run. However, screen or rotary gravure printing techniques are ideally suited to high throughput manufacturing of devices, where only a single design is being used.

It is therefore essential that the ink is robust and reliable such that it can be used in controllable printing devices for research or small scale printing, typically inkjet or bubble jet, but can also be scaled up to be used on different high throughput manufacturing printing processes.

The explosive ink can be supplied to an ink jet printer in an ink cartridge in the usual way.

In one arrangement of a printing system the printer has access to a plurality of explosive inks that can be used in the printing process. In the case of an ink jet printer for example, this can be achieved by providing the inks in separate cartridges, mounted on the carriage of the moveable printing head. Each cartridge has a separate printing nozzle, and can be operated independently of the other cartridges.

In an alternative arrangement the printing system produces an explosive ink in-situ. Accordingly, in a further aspect the present invention provides a method of depositing an explosive formulation, including the steps of:

loading a printing apparatus with a mixture of a binder with at least one metal in the form of particles having a diameter of less than 10 μm and with a mixture of a binder with at least one non-metal, wherein the non-metal is selected from a metal oxide, or any non-metal from Group III or Group IV in the form of particles having a diameter of less than 10 μm such that the at least one metal and/or at least one non-metal mixtures are held separate in the apparatus;

drawing up selected aliquots of the at least one metal and the at least one non-metal mixtures and mixing the same in-situ immediately prior to operation of the apparatus to deposit an explosive formulation onto a substrate.

It may be preferred that the metal and non-metal particles are 1 μm or less in diameter or even are 0.1 μm or less in diameter.

Explosive inks, although insensitive, once they are deposited, still constitute a hazardous material and therefore to reduce the risks of explosion, it may be desirable to locate the explosive ink or constituent parts of the explosive ink in a separate enclosure such that the ink is drawn through a series of pipes such that if an unexpected explosive event occurred during processing only a minimal amount of explosive material would be involved. It may further be the case that systems to prevent flashback of a burning ink may be employed, to decrease the risk of an explosive event escalating.

The explosive device of the present invention may be of various forms and have a variety of applications.

According to a further aspect the invention provides an initiator, wherein the deposited explosive formulation is connected to a heating element, such that, in use the heating element ignites the explosive formulation.

For example, the shape of the pattern in which the explosive ink or formulation is printed on the substrate may be chosen to give desired burn properties, such as the burn time of the explosive ink, or a progressive increase in energy output as the ink burns. The patterns are preferably printed using the explosive ink compositions described above so that a preferred printing technique described above can be used. However they could also be implemented using a formulation and/or other methods of depositing such as brushing, dipping and spraying.

The invention will now be described in more detail, by way of example, and with reference to the drawings in which:

FIG. 1 is an illustration of a device having an explosive ink pattern printed in a zig-zag pattern to form a delay line;

FIG. 2 is an alternative embodiment of a delay line pattern in a spiral shape;

FIG. 3 is an illustration of an explosive device in which the explosive ink is printed in a pattern to give an increasing energy output once ignited;

FIG. 4 is an illustration of a prior art fusehead device;

FIG. 5 is an illustration of an explosive initiator according to the invention; and

FIG. 6 is an illustration of a microthruster array according to the invention.

FIG. 1 shows a simplified initiator component in an explosive device that provides a delay between initiation of the igniter and the energy transfer to the next explosive component.

Initiators are typically the first part of an energetic device and therefore the output from the initiator may need to be increased by adding further increments of a pyrotechnic or other explosive material. The increments between the initiator and the main charge are referred to as an explosive train and this has to provide sufficient energy for the main output charge to be initiated. In this form the deposited explosive formulation is connected to a heating element and to an explosive material such as in use to form an explosive train connecting said element with the explosive material.

The initiator device 20 comprises an elongated explosive ink pattern 22 printed on a substrate 24. The ink can be made and applied to the substrate according to the procedures described above, while the substrate 24 may be any material suitable for the desired application.

A heating element 26 is pre-formed on the substrate, either by lithographic etching or by a metal printing method as set out in the published applications GB0113408 and GB0128571, such that when the explosive ink is printed, one end 28 of the elongated pattern lies over part of the heating element. The heating element is connected to an electrical supply via connections 30; the current flowing through the heating element causes it to be heated to a temperature at which the part 28 of the ink pattern in close proximity to the element ignites.

The explosive ink is printed in a single line which starts adjacent the heating element and terminates adjacent a secondary explosive material (not shown). The end of the line adjacent the explosive material has a patch 32 or wider area of explosive ink. This acts as initiator to the explosive material; the size of the patch being chosen such that the greater output of energy that is released when the patch of ink ignites is sufficient to initiate the next explosive material. The line of ink thus defines an explosive train incorporating an amplifying means.

The line pattern of the ink doubles back on itself repeatedly in a zig-zag pattern such that as one travels along the line, one moves away from the end adjacent the heater element. The zig-zag pattern allows a long length of ink to be printed onto the substrate between the heater element and the patch.

Providing the cross-section of the explosive ink line is kept substantially the same, the time taken for a line of explosive ink to burn from end to end will be reproducible, to within a few percent. Obviously the actual burning time will depend on the nature of the composition, as well as the length of the line. Hence it will be possible by careful selection of the formulation and line length, to control the burn time, such as to produce delays which can be in the range of a few seconds, milliseconds or even microseconds as desired. Therefore a precise and reproducible time can be obtained using a printing technique which allows highly reproducible deposition to take place. In practice, a delay of any amount can be produced merely by changing the length of the line.

For a delay initiator the preferred shape of the explosive pattern is a zig-zag pattern as described above, because it makes most effective use of the area of the substrate on which it is printed, that is, it allows the longest length of line to be printed on a given area. The pattern of explosive ink shown in FIG. 1 is based on the repetition of an S-shaped pattern.

Other patterns are also possible as desired and can be deposited in accordance with the method of the invention. For example a spiral shaped pattern 36 with circular 36 edges, as shown in FIG. 2 can be used or a circular pattern 46 as shown in FIG. 3. In this latter arrangement the heating element is attached at the centre of the circle 32 to produce a burn pattern with an increasing energy output. The versatility of a wet printing technique means that any desired pattern of explosive ink can be readily printed.

The energy output is dictated by the shape of the printed ink pattern and can therefore be made to increase linearly or exponentially in time as desired.

It will be readily understood by person skilled in the art of energetic materials that inert or barrier compositions may need to be inserted between respective lines such that the burning event does not ‘flash’ across to the adjacent line. This barrier coating may be deposited before, during or after the explosive ink or formulation has been deposited.

A particularly important device that can be manufactured as a result of the depositing method provided by the invention is an initiator for a gas generator, wherein the deposited explosive formulation or ink is connected to a heating element and to a gas generating explosive material, such that in use the device forms an explosive train connecting the element with the gas generating explosive material. In use the gas generated by the device can be used to inflate an air bag located in a vehicle, vessel or flying craft.

Additionally the explosive device may form the initiator for a gas generator, wherein the generated gas actuates a seat belt pre-tensioner to restrain a passenger in a vehicle, vessel or flying craft. It will be readily appreciated that additional increments of an explosive material may be required in addition to the formulation, for the correct operation of any gas generating device.

Such systems, particularly those used in cars, increasingly employ a large number of airbags, which are designed to cushion and support the sides and front of a passenger at the time of impact or collision. The inflation of such airbags is effected by the initiation of a small charge. The timing at which the airbags inflate on collision is crucial, otherwise the passenger can suffer damage from the inflation of the airbags themselves, regardless of any damage suffered from the collision.

The preferred device described herein allows the initiation of the charge (which is triggered by the rapid deceleration caused by a crash) and the inflation of the airbags to be orchestrated precisely to within a few microseconds, thereby making a safer protection system for the passenger in the event of a crash.

According to yet a further aspect therefore, the invention provides a method of making a gas generator device for use in a vehicle, vessel or flying craft, comprising the steps of

-   -   a) depositing an explosive formulation or ink according to the         invention onto a heating element;     -   b) placing a gas generating explosive material in intimate         contact with the product of step (a); and     -   c) placing the product of step (b) within a suitable containment         means.

Examples of other applications of the explosive devices of the invention include use in pyrotechnic displays (fireworks), and use in mining applications where precise timing control over the detonation of a number of high explosive charges means that the resulting shockwaves from the explosion can be made to coincide thereby intensifying the force of the blast.

For ignition of certain explosive materials, a minimum amount of kinetic energy in the form of a shock wave from the exploding initiator, as well as heat energy, is required. The ability to print different shapes of initiator patterns means that initiators can therefore be readily produced for use with many different types of explosive. For example the device according to the present invention may be used as part of a safety and arming system in conjunction with explosives which can produce a detonative shock wave, which are capable of initiating high explosive charges.

A further application for the device of the present invention is as a fusehead. In a known fusehead (FIG. 4) the fusehead comb 50 comprises two electrically conductive layers 51, 52 of a material such as copper, separated by an insulator 54, such as laminated fibreboard. A bridgewire 56 is soldered 58 in place connecting the two conductive layers. The comb 50 is dipped into a primary explosive slurry to coat the bridgewire and then repeatedly into a slurry of output composition to build up an initiator bead 60. The output composition is typically a mix of charcoal and potassium chlorate.

To initiate the fuse, a potential difference is applied across the two electrically conductive layers, causing the bridgewire to heat up as it passes a current until it reaches a temperature sufficient to ignite the primary explosive and the output composition of the initiator bead.

The preferred embodiment of the fusehead according to the invention is identical to that shown in FIG. 4, except that the initiator bead 60 is made up solely of explosive ink or formulation according to the present invention.

The metal/non-metal composition of the explosive ink or formulation according to the present invention can be ignited without the need for the application of the additional primary explosive coating on the bridgewire. Thus, the manufacturing process can be made easier and cheaper.

The explosive ink or formulation may also be used to replace the bridgewire in initiator devices. FIG. 5 for example illustrates a glass to metal compression seal 70 suitable for use in an initiator device. FIG. 5 a) shows a plan view of the top of device, and FIG. 5 b) shows a vertical section through the device.

Two metal contacts 71, 72 are formed within a glass moulding 74, such that they are insulated from each other, and provide two exposed electrical terminals on the end face 76 of the device. An explosive ink according to the present invention is applied to the device such that it connects the two terminals.

The explosive ink allows a current to flow from terminal to terminal, until as a result of joule heating, the ink is heated up to its ignition temperature. The explosive ink formed on the device will then ignite, as in known fuses.

Any geometry of pattern of the explosive ink can be applied to the device as long as a connection is established between the two terminals. The firing current required for the device will be dependent on the conduction path offered by the energetic ink. Varying the ink composition, eg: components, particle size, percentage composition, binder, will affect the firing current required.

The terminals could also be provided in a co-axial arrangement.

The printing technique described above can also be used to print an explosive ink for use as an explosive material rather than as an initiator. Such explosives can be printed in patterns like those set out in FIGS. 1 to 3 above.

The printing technique may equally be extended to include high explosives, propellant or initiatory compositions, such that a complete device may be printed, which would include the bridgewire, explosive ink or formulation, optionally a further explosive composition and a main output charge. The explosive train can be selected to produce a burning, deflagration or detonative output depending on the requirements of the output charge.

A particularly important device that can be manufactured as a result of the printing technique provided by the invention is shown in FIG. 6.

FIG. 6 shows a microthruster device for use in airborne devices or apparatus for deployment in space. The microthruster provides very fine control over movement and can therefore be used to manoeuvre or make slight orientation adjustments to the position of satellites, such as geosynchronous satellites.

The microthruster comprises a substrate 80 having a front 81 and a back face. Preferably, the substrate is a ceramic plate. Channels 82 pass through the substrate and have open ends in both the front and back faces. A heating circuit 84 is formed on a ceramic frame 86 which is attached to the back face of the substrate using an epoxy adhesive. The ceramic frame 86 of the heating circuit supports a heating sheet 88 on which individual heating elements 90 corresponding to each of the channels and connections 92 to the heating elements are formed. Thus, an individual heating element is disposed at the bottom of each channel, and can be operated independently of the others.

The channel is filled with an explosive formulation (not shown) which is cured and hardened. The explosive formulation in each channel can be individually ignited by means of its respective heating element. Although an explosive ink could equally be used, there is a chance that the cured explosive could contain traces of solvent, which when subjected to low atmospheric pressure or a vacuum could cause the solvent to evaporate and disrupt the integrity of the cured formulation. Therefore a solventless formulation is preferred.

In use, the substrate is mounted on an apparatus or device with the front face 81 attached to the apparatus. Thus, when the explosive is ignited, the force of the explosion is directed outwards through the heating element sheet, providing a component of thrust in the opposite direction.

The heating sheet and heating elements can be deposited by vapour deposition. As the force of the explosion is directed through the heating sheet, thin heating sheets, made out of polyimide or Nomex paper are preferred.

For small satellite applications, channels may be constructed having diameters in the range 0.25 mm to 1.0 mm. although any diameter hole may be produced The different diameters allow the channels to receive different amounts of explosive formulation, and therefore provide different amounts of thrust on initiation.

In practical applications, the substrate may comprise a plurality of microthrusters, ranging from 100's to many 1000's of these channels, or ‘microwells’, and several microthruster substrates may be attached to the same apparatus. Thus, carefully controlled initiation of the explosive in just one or two of the channels at a time, allows the apparatus to be manoeuvred.

The device manufacturing method of the present invention provides a way of filling the channels with the explosive that is considerably easier and considerably cheaper than existing manufacturing methods. One example of a present technique for manufacturing such apparatuses relies on filling the channel with a loose dry powder and then pressing the material to the required density using a hydraulic press. Invariably, this leads to damage occurring to the heating element or to the substrate which results in wastage, as the whole device has to be thrown away if any of the channels are damaged.

While such manufacturing techniques are acceptable in manufacturing larger one-off devices, they are quite inappropriate for manufacturing a microthruster array of the type described here because of the small size and large number of channels.

The microthruster array may also be produced according to the method of the present invention by printing the explosive formulation directly onto the heating sheet. Layers of the energetic ink may be built up in successive passes of the printing head to deposit the desired amount of explosive material without the need for a substrate with pre-formed channels. However, to prevent the composition from flashing across during burning an inert barrier may be printed/deposited either during or after the explosive formulation has been deposited.

Also, while channels that extend through the thickness of the substrate have been described above, channels which do not extend through the entire thickness but that are more in the form of a recess in the top surface of the substrate may also be used.

Furthermore, although a flat substrate 80 has been used in the example above, any shape of substrate is possible. It could be a curved sheet for example, giving the ability to provide thrust through a range of different angles. A single substrate could also provide a number of exposed faces having cavities for receiving the explosive formulation or ink.

A further application of the device of the present invention is as a security device. It is vital to protect electronic items such as data and electronic hardware/software from fraudulent misuse. Conventional security devices operate by, for example, preventing physical access to the items or by preventing misuse requiring an alphanumeric code or other identification to be input. However, little attention has been given to the use of disabling systems. According to yet further a aspect, the device of the present invention may be in the form of an electronic item printed with an explosive ink or formulation which could be arranged to be activated so ° 30 as to permanently disable the item such that the contents could not be read or used, should the item be misused. For example activation could be brought about by incorrect identification or unauthorised removal of the item, or indeed by a remote means. Conventional explosives would be too hazardous and could not be easily deposited at critical parts of an electronic item to ensure that it was disabled.

It will be readily appreciated by those skilled in the art of energetic initiation that the use of high energy electromagnetic radiation is also capable of providing an initiating stimuli, other than just heat from a bridgewire, such as laser light or microwaves. Therefore it will be clear that the explosive formulation or ink may be initiated by any suitable high energy electromagnetic radiation sources.

A preferred explosive formulation and a method of preparation of the same will now be described.

A preferred explosive formulation comprises aluminium, with a particle size of about 1 to 2 μm, and copper oxide, with a particle size of less than 5 μm, mixed in a 2:3 molar (stoichiometric) ratio. The loading ratio, that is the ratio of the metal and metal oxide powders to the ink binder is such that the binder is present as 30% by volume. The binder itself is a two part epoxy screen printing varnish.

Production of the explosive formulation is carried out by hand as follows: 14.01 g of aluminium powder and 61.86 g of copper oxide are weighed into separate conductive containers. The containers and powder are placed in a fume cupboard to minimise the risk of ignition. As fine aluminium powders are volatile, the powders and containers are also placed behind a blast screen.

The two powders are then added to separate portions of part one of the epoxy screen printing varnish, and the two portions combined and mixed on a high speed mixer to ensure that the metal and metal oxide are well dispersed in the resin. Prior to use, an appropriate amount of part two of the epoxy varnish is added to give a total resin content of 35 g.

The explosive formulation can then be printed onto a substrate using a Viprotech screen printer using a 90 mesh polyester screen, and the printed substrates cured in a convection oven.

Loading ratios in the range 10% to 50% by volume have also been found to produce acceptable explosive ink compositions. 

1. An explosive device comprising a substrate on which one or more explosive formulations is deposited, wherein at least one of the explosive formulations comprises a binder, at least one metal and at least one non-metal, wherein the non-metal is selected from a metal oxide, or any non-metal from Group III or Group IV, wherein the metal and non-metal particles are 10 μm or less in diameter.
 2. An explosive device according to claim 1 wherein the metal and/or non metal particles are 1 μm or less in diameter
 3. An explosive device according to claim 2 wherein the metal and/or non-metal particles are 0.1 μm or less in diameter.
 4. An explosive device according to claim 1 wherein the deposited formulation comprises an insensitive explosive composition having a figure of insensitiveness greater than
 60. 5. An explosive device according to claim 4 wherein the figure of insensitiveness is greater than
 150. 6. An explosive device according to claim 1 wherein the metal is selected from aluminium, titanium or iron.
 7. An explosive device according to claim 1 wherein the non metal is selected from silicon, boron or carbon
 8. An explosive device according to claim 1, wherein the metal oxide is selected from copper oxide, molybdenum oxide or nickel oxide.
 9. An explosive device according to claim 1, wherein the metal is silicon and the non-metal is boron or carbon.
 10. An explosive device according to claim 1, wherein the binder is selected from an ink resin Polyscreen®, Nylobag®, an epoxy resin or urethane.
 11. An explosive device according to claim 1, wherein the binder is an energetic binder.
 12. An explosive device according to claim 11, wherein the energetic binder is selected from Polyglyn (Glycidyl nitrate polymer), GAP (Glycidyl azide polymer) or Polynimmo (3-nitratomethyl-3-methyloxetane polymer).
 13. An explosive device according to claim 1, wherein the binder is present in the range of from 10% to 50% by volume.
 14. An explosive device according to claim 13, wherein the binder is present in the range of from 30% to 40% by volume.
 15. An explosive device according to claim 1, wherein the substrate is an inert substrate such as polyester, polyimide, paper, PET, polystyrene or ceramic.
 16. An explosive device according to claim 1, wherein the substrate comprises a surface of a consolidated explosive material.
 17. An explosive device according to claim 1 and being in the form of an initiator, wherein the deposited explosive formulation is connected to a heating element, such that, in use, the heating element ignites the explosive formulation.
 18. An explosive device according to claim 1 wherein the deposited explosive formulation is connected to a heating element and to an explosive material such as, in use, to form an explosive train connecting said heating element with said explosive material.
 19. An explosive device according to claim 18 wherein the explosive formulation is disposed on the substrate in an elongate pattern having a length which is chosen to provide in use a desired delay time prior to ignition of the explosive material.
 20. An explosive device according to claim 19 wherein the explosive formulation is disposed on the substrate in a spiral or zig-zag pattern.
 21. An explosive device according to claim 17 wherein the deposited explosive formulation is disposed on the substrate in a shape such that when it is ignited by a heating element, the explosive formulation burns with an increasing output of energy until the explosive formulation is exhausted.
 22. An explosive device according to claim 21 wherein the deposited explosive formulation is formed substantially in the shape of a circle, with the heating element located at the centre thereof.
 23. An explosive device according to claim 21 wherein the deposited explosive formulation is disposed on the substrate in a shape that is substantially a segment of a circle with the heating element located at the apex thereof.
 24. An explosive device according to claim 1 wherein the substrate has one or more voids and the explosive formulation fills said voids.
 25. A microthruster device comprising an explosive device according to claim 1, wherein the substrate has a plurality of voids into which the explosive formulation is deposited, further comprising means for selectively igniting said deposits of the explosive formulation to provide thrust.
 26. A microthruster device, according to claim 25 wherein the voids have diameters in the range 0.25 mm to 11.0 mm.
 27. An initiator for a gas generator, comprising an explosive device according to claim 17, wherein the deposited explosive formulation is connected to a heating element and to a gas generating explosive material, such that, in use, the device forms an explosive train connecting said element with said gas generating explosive material.
 28. An initiator for a gas generator according to claim 27, wherein in use the gas generated inflates an air bag located in a vehicle, vessel or flying craft.
 29. An initiator for a gas generator according to claim 28, wherein the generated gas actuates a seat belt pre-tensioner to restrain a passenger in a vehicle, vessel or flying craft.
 30. A pyrotechnic device comprising an explosive device according to claim 1, wherein the deposited explosive formulation further includes one or more Group 1 or Group 2 metal salts to produce, in use, a coloured light or sound emission.
 31. A method of producing an explosive device comprising the steps of: a) mixing a potion of a binder with at least one metal in the form of particles having a diameter of less than 10 μm; b) mixing a further portion of the binder with at least one non-metal, wherein the non-metal is selected from a metal oxide, or any non-metal from Group III or Group IV in the form of particles having a diameter of less than 10 μm; c) mixing together the products of a) and b) to provide an explosive formulation; d) depositing the formulation so produced onto a substrate; and e) causing the formulation to dry on said substrate.
 32. A method of producing an explosive device according to claim 31 wherein the process of deposition in step (d) is by spraying, brushing, dipping or printing.
 33. A method according to claim 32 wherein printing is achieved by a wet printing method.
 34. A method according to claim 33 wherein the wet printing method is selected from ink jet, bubble jet, screen printing or gravure.
 35. A method according to claim 33 comprising loading the explosive formulation into a printing apparatus having a nozzle for spraying a jet of explosive ink.
 36. A method of making an explosive device according to claim 31, wherein the device is a microthruster device for use in space apparatus or airborne craft and wherein the substrate has a plurality of voids which are filled with deposits of the explosive formulation.
 37. A method of making an explosive device according to claim 31, wherein the device is a gas generator device for use in a vehicle, vessel or flying craft, wherein the substrate in step (d) of claim 31 is a heating element and wherein the method further comprises the steps of: f) placing a gas generating explosive material in intimate contact with the product of step (d); and g) placing the product of step (f) within a suitable containment means.
 38. A method of depositing an explosive formulation, including the steps of: a) loading a printing apparatus with a mixture of a binder with at least one metal in the form of particles having a diameter of less than 10 μm and with a mixture of a binder with at least one non-metal, wherein the non-metal is selected from a metal oxide, or any non-metal from Group III or Group IV in the form of particles having a diameter of less than 10 μm such that the at least one metal and/or at least one non-metal mixtures are held separate in the apparatus; b) drawing up selected aliquots of the at least one metal and the at least one non-metal mixtures and mixing the same in-situ immediately prior to operation of the apparatus to deposit an explosive formulation onto a substrate.
 39. A method of depositing an explosive formulation according to claim 38 wherein the metal and non-metal particles are 1 μm or less in diameter.
 40. A method of depositing an explosive formulation according to claim 39 wherein the metal and non-metal particles are 0.1 μm or less in diameter.
 41. An explosive ink comprising an explosive formulation as described in claim 1, together with a volatile organic solvent.
 42. An explosive ink according to claim 41 wherein the volatile organic solvent is selected from a lower alkyl alcohol, ketone or ether or from petroleum ethers ranging from C5 to C10.
 43. An explosive ink according to claim 41, wherein the metal and/or non-metal particles are 1 μm or less in diameter.
 44. An explosive ink, according to claim 43, wherein the metal and/or non-metal particles are 0.1 μm or less in diameter. 